The present specification generally relates to diffractive optical elements. More particularly, the present specification describes diffractive optical elements on non-flat substrates using electron-beam lithography.
The elements that shape a propagating wavefront are a key part of any optical system. Diffractive optical elements offer major advantages over conventional refractive optical elements in terms of size, weight, and cost. Bulky groups of classical optical elements, such as lenses, mirrors, beamsplitters and filters, are replaced by a single, planar diffractive optical element. As a result, optical systems can be made smaller, more robust and less expensive. In addition, these devices can perform complex waveshaping functions that are often beyond the capabilities of conventional elements.
A diffractive optical element is a complex pattern of sub-micron structures which can modulate and transform light in a predetermined way. The element utilizes ultra-precision surfaces that have a series of zones, which have extremely small steps at the zone boundaries. The placement of these zones allows an optical designer to precisely shape the emerging optical wavefront. The required step heights at the zone boundaries are typically between 1 and 10 xcexcm.
A tool with very high resolution and large flexibility is needed to manufacture micro-structures with arbitrary shapes. A direct-write electron beam lithography system can be used to fabricate diffractive optical elements. In electron beam lithography, a resist is exposed with a focused beam of electrons. A computer-program converts the calculated phase data to electron beam exposure doses. The beam diameter can be focused down to about 10 to 50 nm.
The present disclosure describes diffractive optical elements having diffraction gratings on curved surfaces with electron beam lithography. The curved surface can act as an optical element to produce flat and aberration-free images in imaging spectrometers. In addition, the fabrication technique can modify the power structure of the grating orders so that there is more energy in the first order than exists for a typical grating. The inventors noticed that by using electron-beam lithography techniques, a variety of convex gratings that are well-suited to the requirements of imaging spectrometers can be manufactured.
The present disclosure describes an optical grating having a non-flat substrate. A film of resist is coated on top of the substrate, e.g., a plano-convex lens, using a standard semiconductor fabrication spin-coater. The resist is etched to form at least one region of blazed surface. Each region preferrably has a blazed surface with same blaze angle.
The substrate is made of glass, aluminum, or both. The curvature or the sphericity of the substrate is low. In a preferred embodiment, the film of resist is made of a special Plexiglass material called polymethyl methacrylate xe2x80x9cPMMAxe2x80x9d. The film of PMMA has a thickness of about 2 to 3 xcexcm.
In a particular embodiment, the film of resist is etched to form a single region of blazed surface such that the blaze angle stays constant with respect to the local grating surface. In another particular embodiment, the film of resist is etched to form a grating with two concentric blaze regions having a middle region and an outer ring region such that the total grating area is split into two concentric regions with different blaze angles. The middle region occupies approximately 33% of the total area and the outer ring region the remaining 67%. In a preferred embodiment, the film of resist is etched to form a grating with a groove having two segments. The two segments share a common region. Each segment has a different slope to broaden the second order wavelength response.
In addition, this device allows two different kinds of modification to the incoming light, focusing from the lens part and diffraction from the grating parts.
In a preferred embodiment, the etching means used is an electron-beam fabrication technique. The tool used in the electron-beam technique is about 50 kV, 2 mA, 0.5 xcexcm beam.
The present disclosure also includes a method of manufacturing an optical grating. The method includes laying a curved substrate, coating a film of resist on top of the curved substrate using a standard semiconductor fabrication spin-coater, and etching the resist to form at least one region of blazed surface. The method of etching the resist on a curved surface include predetermining focus, X- and Y-deflector gain calibration, and X- and Y-deflector rotation calibration of an electron-beam writer. The method also includes laying down calibration marks on said curved surface, and exposing the grating pattern on the curved surface.
The present disclosure also includes an imaging spectrometer that has an input slit for passing beams of light, and a set of mirrors. A first mirror is used to reflect the beams of light. A second mirror is an optical grating described above. A third mirror is used to reflect a wavefront of light formed by the optical grating. The spectrometer also includes a plate for processing the spectrum of wavefront of light reflected by the third mirror. In a preferred embodiment, the spectrometer is of an Offner type. This Offner spectrometer includes an input slit that is 1 cm long and a grating that covers the wavelength band of approximately 0.4 to 1.0 xcexcm in the second order and approximately 1.0 to 2.5 xcexcm in the first order. The grating in the spectrometer has a substrate curvature of approximately 0.0146 mm-xe2x88x921. The spectrometer also has an angle of incidence of approximately 25 degrees and a pitch of about 20.7 xcexcm.
The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other embodiments and advantages will become apparent from the following description and drawings, and from the claims.